The present invention relates generally to a controller for administering a hydrostatic sterilization process being performed on line of carriers that each carry a set of containers. In particular, it pertains to such a controller that also provides on-line handling of a deviation in a scheduled parameter during the process by identifying any carriers with under processed containers as a result of the deviation.
A hydrostatic sterilization system is a continuous source processing system. It is widely used in the canning industry to sterilize a shelf stable food product packaged in containers. It is used most often for sterilizing a food product that does not benefit from mechanical agitation of containers, as is performed in a rotary sterilization system.
A hydrostatic sterilization system comprises a hydrostatic sterilizer that has a conveyor (or chain) and a line of carriers {1, . . . , i, . . . , I}line conveyed by the conveyor. Each carrier i carries a set of containers and is conveyed through the hydrostatic sterilizer by the conveyor. The conveyor has a scheduled conveyor speed for conveying the carriers through the hydrostatic sterilizer. Moreover, the containers carried by each carrier are treated with scheduled retort temperatures in the hydrostatic sterilizer.
In order for the food product in the containers of each carrier i to be commercially sterilized, a total lethality Fi over a total time interval [tf,i, td,i] that satisfies a predefined target total lethality Ftarg must be delivered during the hydrostatic sterilization process to the product cold spot of each container of the carrier. Here, tf,i and td,i are the loading and unloading times when the carrier is loaded into and unloaded from the hydrostatic sterilizer. The target total lethality is set by the USDA (U.S. Department of Agriculture), the FDA (Food and Drug Administration), and/or a suitable food processing authority for destroying certain microorganisms. The conveyor speed and the retort temperatures are then scheduled so that the containers of each carrier will receive a time-temperature treatment according to a scheduled time-temperature profile that delivers a total lethality to the containers which satisfies the target total lethality.
As is well known, the lethality Fi delivered to each container in a carrier i over a particular time interval [tm, tk] is given by the lethality equation:       F    i    =            ∫              t        m                    t        k              ⁢                  10                              (                                                                                T                    cs                                    ⁡                                      (                    t                    )                                                  i                            -                              T                REF                                      )                    /          z                    ⁢              ⅆ        t            
where tm and tk are respectively the begin and end times of the time interval [tm, tk], TCS(t)i is the product cold spot time-temperature profile for the container, z is the thermal characteristic of a particular microorganism to be destroyed in the sterilization process, and TREF is a reference temperature for destroying the organism. Thus, the total lethality Fi delivered to the container over the total time interval [tf,i, td,i] due to the scheduled retort temperatures is given by this lethality equation, where tm=tf,i and tk=td,i.
The total time interval [tf,i, td,i9  and the product cold spot time-time-temperature profile TCS(t)i must be such that the total lethality Fi over [tf,i, td,i] satisfies the target total lethality Ftarg. In order to ensure that this occurs, various mathematical simulation models have been developed for simulating the product cold spot time-temperature profile based on the scheduled retort temperatures. These models include those described in Ball, C. O. and Olson, F. C. W., Sterilization in Food Technology; Theory, Practice and Calculations, McGraw-Hill Book Company, Inc., 1957; Hayakawa, K., Experimental Formulas for Accurate Estimation of Transient Temperature of Food and Their Application to thermal Process Evaluation, Food Technology, vol. 24, no. 12, pp. 89 to 99, 1970; Thermobacteriology in Food Processing, Academic Press, New York, 1965; Teixeira, A. A., Innovative Heat Transfer Models: From Research Lab to On-Line Implementation in Food Processing Automation II, ASAE, p. 177-184, 1992; Lanoiselle, J. L., Candau, Y., and Debray E., Predicting Internal Temperatures of Canned Foods During Thermal Processing Using a Linear Recursive Model, J. Food Sci., Vol. 60, No. 4, 1995; Teixeira, A. A., Dixon, J. R., Zahradnik, J. W., and Zinsmeister, G. E., Computer Optimization of Nutrient Retention in Thermal Processing of Conduction Heated Foods, Food Technology, 23:137-142, 1969; Kan-Ichi Hayakawa, Estimating Food Temperatures During Various Processing or Handling Treatments, J. of Food Science, 36:378-385, 1971; Manson, J. E., Zahradnik, J. W., and Stumbo, C. R., Evaluation of Lethality and Nutrient Retentions of Conduction-Heating Foods in Rectangular Containers, Food Technology, 24(11):109-113, 1970; Noronha, J., Hendrickx, M., Van Loeg, A., and Tobback, P., New Semi-empirical Approach to Handle Time-Variable Boundary Conditions During Sterilization of Non-Conductive Heating Foods, J. Food Eng., 24:249-268, 1995; and the NumeriCAL model developed by Dr. John Manson of CALWEST Technologies, licensed to FMC Corporation, and used in FMC Corporation""s LOG-TEC controller.
However, if any of the actual retort temperatures in the hydrostatic sterilizer drops below a corresponding scheduled retort temperature, a temperature deviation occurs. Traditionally, when such a deviation occurs, the controller stops the conveyor and prevents any of the carriers {1, . . . , i, . . . , I}line from being loaded into or unloaded from the hydrostatic sterilizer until the deviation is cleared. But, this approach causes numerous problems. For example, significant production down time will result. And, many carriers { . . . , i, . . . }ovrpr will have over processed containers since the total lethalities { . . . , Fi over [tf,i, td,i], . . . }over actually delivered to these containers will significantly exceed the target total lethality Ftarg. All of these problems may result in severe economic loss to the operator of the hydrostatic sterilization system.
In order to prevent such loss, a number of approaches have been discussed and proposed for on-line control of sterilization processes. However, all of these approaches concern control of batch sterilization processes performed on a batch of containers {1, . . . , i, . . . , I}batch. In a batch sterilization process, all of the containers generally receive the same time-temperature treatment whether or not a temperature deviation occurs. Thus, when a deviation does occur, a correction to the process can be made which simultaneously effects all of the containers so that a minimum total lethality Fi over [tb, te] will be delivered to each container i, where tb and te are the begin and end times of the batch sterilization process. An example of such an approach is described in concurrently filed and co-pending U.S. patent application Ser. No. 09/187,333, entitled Controller and Method for Administering and Providing On-Line Correction of a Batch Sterilization Process , filed on Nov. 6, 1998, with Weng, Z. as named inventor. This patent application is hereby explicitly incorporated by reference.
In contrast, each carrier i in a hydrostatic sterilization process will receive a unique time-temperature treatment. Thus, the total lethality Fi over [tf,i, td,i] actually delivered to each carrier""s containers is different. This makes it difficult to identify, while on-line and in real time, each carrier that will have a total lethality predicted to be delivered to its carriers that is below the target total lethality Ftarg. As a result, the development of a controller that provides on-line handling of a temperature deviation in a hydrostatic sterilization process without stopping the conveyor has been inhibited.
However, in Weng, Z., Park, D. K., and Heyliger, T. L., Process Deviation Analysis of Conduction Heating Canned Foods Processed in a Hydrostatic Sterilizer Using a Mathematical Model, Food Processing Automation Conference IV, FPEI, ASAE, pp. 368-379, 1995, the distribution of the total lethalities F1 over [tf,1, td,1], . . . , Fi over [tf,i, td,i], . . . , FI over [tf,i, td,I] actually delivered for carriers {1, . . . , i, . . . ,I}line was studied when a temperature deviation occurred. From this, an off-line approach was proposed to identify each carrier i with a total lethality Fi over [tf,i, td,i] that was actually delivered which is less than the target total lethality Ftarg. But, this approach is not performed in real time and limited to the conditions of a single temperature deviation and a well controlled water level in the sterilization chamber of the hydrostatic sterilizer.
In summary, the present invention comprises a hydrostatic sterilization system, a controller for use in the hydrostatic sterilization system, and a method performed by the controller. The system, controller, and method are used to administer a sterilization process performed on a line of carriers and provide on-line handling of a deviation from a scheduled parameter during the process. The carriers carry containers of a shelf stable food product that is to be sterilized in the sterilization process. In addition to the controller, the hydrostatic sterilization system includes a hydrostatic sterilizer.
The controller controls the hydrostatic sterilizer to perform the hydrostatic sterilization process according to scheduled parameters. When a deviation in a specific one of the scheduled parameters occurs, the controller identifies those of the carriers that will in response have a total lethality predicted to be delivered to them during the sterilization process that is less than a predefined target lethality. This specific scheduled parameter may be a scheduled retort temperature in a chamber of the hydrostatic sterilizer through which the line of carriers is conveyed. This specific scheduled parameter may also be a scheduled water level in a sterilization chamber of the hydrostatic sterilizer through which the line of carriers is conveyed. Or, it may be a scheduled initial product temperature for the containers in the carriers or a scheduled conveyor speed for conveying the carriers in line through the hydrostatic sterilizer.